the snow as soon as possible. Frequent dragging
tive cooling at the surface, and conduction to the
and planing will accomplish this and will limit
free surface. Different surface material types will
the solar heating of individual sand particles.
have different properties which cause them to
respond uniquely with regard to these factors.
During initial reconnaissance of the site, fea-
tures in the region of the Pegasus runway alerted
PATCHING OF
us to the fact that natural melting (both surface
ICE DAMAGE AND FLAWS
and subsurface) was not uncommon at this site.
Whenever cracks, gouges, broken or weak ice
In addition, subsurface melt pools are reported in
are discovered on the runway they should be
the literature (Paige 1968, Mellor and Swithinbank
patched as soon a practical. Patching is best per-
1989). During the 199192 summer season, for-
formed when the air and ice are cold (less than
mation of a subsurface melt pool was witnessed
10C). However, if the damaged or flawed area
by the Pegasus construction team. Paige's dis-
appears to represent a safety hazard to aircraft, it
cussion is limited to speculation on the physics
should be repaired immediately. Even when con-
involved in melt pool formation, since he did not
ditions are warmer than desirable, the procedures
document the actual formation process. Subsur-
outlined in the patching section will be effective
face melting could occur due to the absorption of
in fixing the ice. (When patching small areas dur-
radiation at some depth in the ice by a foreign
ing warm and sunny periods, it may be advanta-
substance (dirt or rocks) or by the trapping of
geous to shade the patch site from the sun,
emitted longwave radiation in an air bubble. The
provided there is plenty of allowance for air flow.)
most likely mechanism for subsurface melting is
the convective cooling of the ice surface such that
absorption of radiation and heating are occurring
at some depth in the ice instead of initiating at the
PREVENTING OCCURRENCE
surface. The presence of absorbing media or
OF MELT FEATURES
bubbles would then serve to enhance and acceler-
Natural surface and subsurface melting are not
ate the process of subsurface melting.
singularly attributable to the local ambient tem-
By trial, we discovered that natural melt fea-
perature. The potential for melting is a complex
tures could be prevented at the Pegasus site by
combination of the angle of inclination of incom-
completely covering exposed ice surfaces with
ing radiation (which directly determines the am-
30 cm (12 in.) of snow at a density of 0.350.45
bient temperature), the amount of reflected
balance at the site. Our field results were easily
radiation (attributable to the surface characteris-
confirmed by calculating radiation transmission
tics), the absorption and transmission of the
in snow. Shortwave radiation penetrates the snow-
nonreflected radiation, the magnitude of convec-
1.0
0.45 g/cm3
0.35 g/cm3
0.8
0.25 g/cm3
0.6
0.4
0.2
Figure 76. Ratio of shortwave solar
radiation at depth x in the snowpack
(Qx) to shortwave solar radiation
0
100
200
300
400
500
600
absorbed at the snow surface (Qa).
Snow Depth (mm)
72
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